mid-15c., "ample, profuse," from Old French frequent, or directly from Latin frequentem (nominative frequens) "often, regular, repeated; in great numbers, crowded, numerous, filled, full, populous," which is of uncertain origin. Watkins says probably from PIE *bhrekw- "to cram together," and compares Greek phrassein "to fence in," Latin farcire "to cram," But Beekes regards the connection to the Greek word as "quite uncertain." Meaning "common, usual" is from 1530s; that of "happening at short intervals, often recurring" is from c. 1600.
"verb which expresses repetition of action," 1520s, from French fréquentatif, from Late Latin frequentativus "serving to denote the repetition of an act," from Latin frequentat-, past-participle stem of frequentare "visit regularly; do frequently, repeat," from frequentem (see frequent (adj.)). Frequentive is considered incorrect, because -ive adjectives are normally formed on the Latin past participle.